State police have said that Jeffrey Giuliano grabbed his gun and went outside at about 1am on September 27 2012 when his sister called to say someone was trying to break into her house next door.

Authorities say Mr Giuliano saw a masked person holding a knife come toward him in a threatening manner and shot him several times. He later was told the person was his son Tyler.

State attorney Stephen Sedensky III's findings marked the conclusion of his investigation into the shooting in New Fairfield, a town of nearly 14,000 about 50 miles north of New York City.

Mr Giuliano thought the masked person had a gun and feared for his life, his attorney Gene Zingaro, has said. He later learned it was a knife.

The attorney said Mr Giuliano had heard reports of a break-in and a sexual assault in the same town, and had been worried enough to call a family meeting to make sure his children took precautions, he said. On the night of the shooting, he shouted at the masked person but did not hear a response.

"In my estimation, Jeff Giuliano felt like he had happened upon maybe the same intruder," Mr Zingaro said.

Mr Giuliano and his wife adopted Tyler and his sister about four years before the shooting. The children would have gone into foster care if the couple, who had three other children, had not adopted them, Mr Zingaro said.